Electrostatic spray systems suitable, for example, for painting automobile bodies, have existed for some time as disclosed in European Pat. No. 9,032,391. This has been accomplished by applying a high voltage potential to the spray head of a spraying apparatus in order to produce an electrical field between the spray head and the grounded object to be coated to charge the sprayed coating material.
The problem with such spraying apparatus is that when a coating material of relatively good conductivity is used, such as a so-called water enamel, the insulation resistance through the line connecting the spray head to the paint storage system is too low for a storage system at ground potential. This problem may be solved by insulating the entire storage system from the ground potential. However, this is undesirable, especially if, when changing colors, the storage system comprises a plurality of storage tanks. Apart from the considerable cost of insulating, the capacity of an extensive storage system may be so large that the corresponding charging energy (1/2 CU.sup.2) may be too high, creating the danger of an explosive discharge at the spray head. Moreover, even if largely solvent-free paint is used, this danger cannot be completely eliminated. Further the tanks which are at a high electrical potential cannot be topped up without switching off the power, unless costly additional equipment, in the form of intermediate tanks, or the like, is used, as disclosed in German Pat. No. 2,900,660. Also, many of these known systems are expensive since they utilize uneconomical high power voltage sources.
The subject invention is related to an apparatus as disclosed in U.S. Pat. No. 3,393,662 in the name of Ronald J. Blackwell. His apparatus has a grounded rotary atomizer and a plurality of externally charging electrodes arranged circumferentially around the spraying device and connected to a high voltage source.
The problem with such spraying devices is that it is very difficult to protect the spraying device and charging electrodes from rapid contamination by paint particles due to a substantially radial spraying component from the spray head. These contaminated particles not only impair the electrical field, but after a color change, may also contaminate the article to be coated when subsequently used. Moreover, a possibility of the painting efficiency being substantially lower than that of conventional devices having their spray head connected to a high voltage potential exists.